The present invention relates to a drill bit for drilling holes in a wood workpiece and, more particularly, to a drill bit for drilling holes having a diameter of more than 10 mm.
A known drill bit such as, for example, the drill bit disclosed in U.S. Pat. No. 2,358,077, is configured as a special tool piece and includes a conical tip having a thread series operable to draw or thread in the drill bit into the wood workpiece, as well as a cylindrical portion provided with a spiral screw shaped splinter groove and having a relatively large diameter. A substantially radial vertically extending main cutting portion is located at the area of the beginning of the cylindrical portion and this main cutting portion is followed by an axially extending side cutting portion of the cylindrical portion extending at an angle to the main cutting portion.
Other known drill bits of this type, which are disclosed in U.S. Pat. No. 2,883,888, U.S. Pat. No. 3,180,379, and U.S. Pat. No. 3,064,699, additionally include a thread series in the cylindrical portion which cuts a threaded wall in the hole being drilled and guides the drill bit along the hole. Since this threaded wall remains in the hole after the hole has been drilled to the desired depth, it is only possible to withdraw the drill bit from the now completed hole by a reverse threading movement of the drill bit and this process disadvantageously adds to the time for the drilling operation. Moreover, as noted, the wall of the hole is not smooth surfaced but has, instead, threads.
The noted drill bits are used in many situations and can have a length up to 450 mm. A disadvantage of these known drill bits, as well as of the typical spiral drill bits, is that they break through the backside of the wood workpiece being drilled in a manner of impact which produces splinters, whereby the backside of the wood workpiece does not have a hole with a clean lip.
German Patent Publication G 90 01 633.5 discloses a cutting tool piece having a cutting portion 1 which has three cutting angles 3 and three cutting grooves 4. The three cutting angles 3 lie on a conical sleeve surface.
The present invention provides a drill bit which solves the challenge of providing an improved drill bit which permits easy manufacture thereof and avoids to the greatest extent possible a breaking through of the backside of the wood workpiece being drilled as well as the challenge of providing a drill bit which minimizes the magnitude of the drill penetration force which needs to be applied to the drill bit during the drilling operation.
To solve the challenge of providing such a drill bit, the present invention provides a drill bit having a conically shaped tip with at least one main cutting portion and having a thread series which draws the drill bit into the wood workpiece, with the conically shaped tip having a diameter the same as the selected diameter of the hole to be drilled in the wood workpiece. The thread series of the conically shaped tip reduces the drill penetration force which needs to be applied to the drill bit, not only because the thread series operates to draw or thread itself into the wood workpiece, but also because the use of the thread series results in many fine cuttings or shavings which are virtually in the form of saw cuttings. Since the thread series exerts a constant pulling along effect on the conically shaped tip as the drill bit pierces the backside of the wood workpiece, there is no sudden breaking through of the backside of the wood workpiece; instead, the wood in this area is cut into small cuttings, thereby ensuring a clean lip on the hole on the backside of the wood workpiece.
In an advantageous configuration of the drill bit of the present invention, the drill bit includes a cylindrical, non-threaded portion operable to guide the drill bit into the hole, the cylindrical, non-threaded portion transitioning into the conically shaped tip and having a side cutting portion.
The drill bit of the present invention can include at least one axially extending, preferably spiral screw shaped, splinter groove.
In a particularly advantageous embodiment of the drill bit of the present invention, the conically shaped tip is provided with a central drill portion of a relatively small diameter extending in the direction of the drill penetrating movement of the drill bit, and the central drill portion has a thread series having the same steepness or slope gradient as the thread series of the conically shaped tip. In the event that the thread series of the central drill portion is configured as a self cutting thread series, it is possible for the central drill portion to bore into the wood without any special force application on the drill bit, and the thread series of the central drill portion reinforces in this manner the operation of the thread series of the conically shaped tip in drawing in the drill bit into the hole without the need to apply a special drill penetrating force to the drill bit.
If the thread series of the conically shaped tip is configured with a smaller steepness or slope gradient than the thread series of the central drill portion, or if the thread series of the conically shaped tip is double threaded, an advantageous size reduction is achieved with respect to the cuttings produced by the conically shaped tip and this facilitates the carrying away of the cuttings.
The central drilling portion can, in the same manner as the conically shaped tip and the cylindrical portion, be provided with at least one spiral screw shaped splinter groove which can be configured as an extension of the splinter groove in the conically shaped tip and the cylindrical portion.
The central drilling portion preferably includes a self cutting thread series so as to eliminate the need to drill a locating hole to set the drill bit in its initial drilling position.
If the drill bit is to be used to drill a blind hole in the wood workpiecexe2x80x94a hole extending only partially through the wood workpiecexe2x80x94instead of a through hole, it is necessary to reverse the rotation direction of the drill bit upon reaching the desired depth. In this connection, the drill bit of the present invention is moved in a reverse withdrawal direction to withdraw it from the hole in the wood workpiece, which is an operation that cannot be performed with conventional drill bits without additional modification. This reverse movement of the drill bit can be facilitated by a conical transition portion which extends from the conically shaped tip or the cylindrical portion to a shaft of relatively small diameter, whereby the conical transition portion is provided with a thread series having the same thread handedness as the thread series of the conically shaped tip. If, on the other hand, the thread series of the conical transition portion has a thread handedness opposite to the thread handedness of the thread series of the conically shaped tip, it is not necessary to reverse the rotation of the drill bit to withdraw the drill bit from the hole. The steepness or slope gradient of the thread series of the conical transition portion can be the same or different than the steepness or slope gradient of the thread series of the conically shaped tip.